This is a terminology issue only. In the business, "critical" means at power equilibrium where K-effective = 1 and power is stable.I can't reconcile that -- a view of criticality as a continuum -- to the Slotin accident at Los Alamos (criticality by fleetingly and accidentally joining the two halves of the demon core) and the Cecil Kelley centrifuge accident (criticality by centrifuge).
In the case of both your cited accidents, they were dealing with fast fuel, processed and shaped (Slotin), and in-processing/not formed (Kelley). This would imply prompt criticality events which is a super-critical condition demonstrated by rapid uncontrolled power rise. They both made mistakes regarding the mass/geometry, Slotin probably knowing he was playing with fire, whereas Kelley not so much. In any event, the power transient in both cases required up-power, which by definition is "super-critical". It doesn't really matter to me how it is portrayed in media, beit wikipedia, movies, or news articles, that is for laymen and zombies.
To be fair, the term "criticality" actually encompasses all three conditions, sub, stable, and super. It not just limited to stable, or to super.
But be sure, that in both accidents, the fuel masses were in supercritical conditions, and it is doubtful that they stabilized. A rapid up then a rapid down.
Think of it like playing with a light dimmer where you spin it from off to max and then back to off.
If you want to read up on a really fun prompt critical accident, read up on SL-1. It was part of our former "expeditionary nuclear power" program back in the day. I have been to the former site, sobering. I think there are some videos floating around somewhere on it as well. I like to call it the "don't play with control rods unless you are sure" accident. Kind of similar to Chernobyl.